Kate St. Vincent Vogl first learned she should be ashamed when her sister got into a fight with a neighbor boy who retaliated, "Yeah, well, you're adopted!"

Her sister ran home and confronted their mother, who admitted it was true. The news that both were adopted stunned Vogl, now a 43-year-old writer from Minneapolis.

"I took one look at her blond hair and blue eyes and said, 'Maybe you are, but I'm not,'" she said. "I'd seen Sesame Street. I knew which one didn't belong. It was as if I knew already at age 8 that being adopted was not something you'd want to be."

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Vogl is one of an estimated 6 to 8 million Americans who are adopted. Many now tell the Evan B. Donaldson Institute that they were stigmatized as children and struggled with their identity and self-esteem well into adulthood.

"This is the biggest and deepest study of its kind," he told ABCNews.com. "There are big universal truths and we are finally getting at some of them to help us be better parents, better professionals and people who grow up to be healthy human beings."

Many of the 468 adult respondents said they experienced discrimination. More white adoptees (35 percent) than Korean (21 percent) indicated teasing simply because they were adopted.

About 86 percent said they had taken steps to find their birth parents and that finding them was the single factor that helped them gain a positive adoptive identity.

"We would never deride people based on religion or a handicap or another piece of who somebody is," Pertman told ABCNews.com. "But that's what too often happens to people in the adoption world."

"There is institutional discrimination in our culture," Pertman said. "We watch TV shows and some kid is adopted and they'll say he's a bad seed and suspect from the get-go. Or when people ask, 'What happened to his real parents?' How do you think that makes kids feel?"

Pertman hopes the study will help promote laws, policies and practices to give adoptees greater access to information about their birth and to help erase stereotypes to improve their lives.

For Vogl, knowing she was adopted didn't change the love she felt for her family, but she also learned it was the "elephant in the corner, not to be talked about.

"I suppose you could say that adoption-related discrimination began from the moment adoption became a part of my life," she said. "My sister then dialed up all of her friends to confess she was adopted, like it was some sort of terminal disease."

The study also revealed that only 45 percent who sought out their birth parents were successful.

Adoptees told ABCNews.com that they were largely stymied by state laws that guard the secrecy of birth certificates and adoption records or the expense of hiring searchers.

Some were also rejected by their birth parents.

Secret Shame of Adoption

The secret surrounding her own adoption took a devastating toll on Joan Wheeler, a disabled social worker from Buffalo, N.Y. The last of five children born to a married couple, she was adopted as an infant when her mother died of cancer.

"Distant relatives of deceased mother kept it as a secret, raising me as their only child," she told ABCNews.com. "I knew I was adopted, but I didn't know I had brothers and sisters. I am still paying for it."

Her siblings sought a reunion when she was 19, but dark secrets and jealousies prevented them from bonding.

Now 53 and a disabled social worker, Wheeler has a troubled relationship with both her adoptive and biological families, who have chastised her for openly talking about the family's past in her upcoming book, "Forbidden Family."

Her identity struggle was so painful, she doesn't even believe in adoption, wishing she had been placed in foster care or a guardianship so she could have had access to her blood relatives.

"Babies are not commodities, they have lives and identities," Wheeler said.

Today, birth parents often welcome contact, but before the 1970s and 1980s closed adoptions were the norm and records were sealed tightly at birth.

Now, experts see those policies as archaic, robbing adoptees not only of vital medical records, but of their sense of identity.

Only Alaska, Kansas, New Hampshire, Maine, Oregon, Alabama, Tennessee and Delaware have broad access to original birth certificates.

"I feel like it's the last civil rights struggle," she told ABCNews.com. "There is a bias toward the adopters who are looked on as saviors. They are the ones that get attention at the expense of the adoptee and their birth relatives. We don't count."

Without her original birth certificate, Warshaw, author of "A Legitimate Life," said she feels like she has a "fake identity."

She was adopted in as an infant in 1947 after her birth mother had a "revenge affair" during a separation from her philandering husband.

"The shame forced her into secrecy," Warshaw said of her birth mother, who turned to the Cradle Adoption Agency in Chicago where wealthy parents like Bob Hope adopted.

"Her sister drove her under cover of darkness to the agency where she stayed until she gave birth to me," she said.

Warshaw began a complicated search for her birth parents that eventually led her to the Chicago adoption agency, where she was told, "Sorry, we can't tell you anything."

"It was against the law for me to find out who I was, and it just drove me insane that I wasn't allowed to find out even my name or ethnicity," she said.

"It was just cruel and many adoptees feel like we are aliens and don't belong," said Warshaw. "We're just lost."

In 1980, at the age of 33, she found her mother.

"She was ridden with shame and feared what friends, family and community would think," said Warshaw. "After several conversations with her and hearing her cold-hearted words of rejection, I ended my attempts to truly be her daughter."

Eventually, Warshaw was able to connect with her half brother, an artist like herself. And this summer, she met 12 third cousins in Iowa.

"It was like I flew home," she said. "It was so healing and wonderful. All the women had the same body style and walk and eyes. I feel totally complete."

Reunions are challenging, according to Craig Hyman, a 51-year-old adoption triad life coach from New York City who found both his biological parents 27 years ago.

""But the richness and advantages far outweigh the complications along the road."

Buvetta Bryant struggled with her identity her whole life, before a reunion with siblings this year at the age of 67. "I felt a part of me was missing, an empty feeling."

Now she has met some of the 10 siblings on her birth mother's side and six more from her father. "This has been an experience of a lifetime."

She was adopted in 1950 and given an amended birth certificate naming her adoptive parents as birth parents. By 1991, she searched and found her birth mother, with whom she has a "positive relationship."

"Telling adoptees about their history validates their experience, that they were there," Russell told ABCNews.com. "Only in adoption is it considered strange or abnormal to want to know where you come from.

"Searching for birth family is not a statement about adoptive parenting. It is a statement of wanting to know one's self."

Reunions With Birth Families Positive

Many adoptees and adoptive parents feel more connected after a reunion with the birth family, according to both Russell.

Such was the case with Kate Vogl, who had no desire to connect to her biological parents -- "a couple of college students who got into trouble."

But late one night, at 28, she got a call from her birth mother -- just two months after the death of her adoptive mother.

"She found me through my mother's obituary," said Vogl, who remembers her father's reaction to the reunion: "There's always enough love to go around."

"I'm not sharing holidays," Vogl insisted. "I was really reluctant at first because of my strong loyalty to my adoptive mother."

"I know she would like me to call her mother, but she's not my mom" said Vogl. "The wonderful thing about this is I've lost my mom and I've had Val, who is a wonderful person, come in to my life."

Though she sees discrimination as a "loaded word," she said knowing the negative experiences of adoptees can help educate society about their struggle for identity.

"I don't think that the instruction needs to be on adoptive parents' side, but society as a whole," Vogl said. "The thing that really gets to me is how people feel that the bonds formed in adoptive families are not as strong as the bonds with biological families.